DESCRIPTION

The functions tsleep(), msleep(), msleep_spin(), pause(), wakeup(), and
wakeup_one() handle event-based thread blocking. If a thread must wait
for an external event, it is put to sleep by tsleep(), msleep(),
msleep_spin(), or pause(). Threads may also wait using one of the
locking primitive sleep routines mtx_sleep(9), rw_sleep(9), or
sx_sleep(9).
The parameter chan is an arbitrary address that uniquely identifies the
event on which the thread is being put to sleep. All threads sleeping on
a single chan are woken up later by wakeup(), often called from inside an
interrupt routine, to indicate that the resource the thread was blocking
on is available now.
The parameter priority specifies a new priority for the thread as well as
some optional flags. If the new priority is not 0, then the thread will
be made runnable with the specified priority when it resumes. If
priority includes the PCATCH flag, signals are checked before and after
sleeping, otherwise signals are not checked. If PCATCH is set and a
signal needs to be delivered, ERESTART is returned if the current system
call should be restarted if possible, and EINTR is returned if the system
call should be interrupted by the signal (return EINTR).
The parameter wmesg is a string describing the sleep condition for tools
like ps(1). Due to the limited space of those programs to display
arbitrary strings, this message should not be longer than 6 characters.
The parameter timo specifies a timeout for the sleep. If timo is not 0,
then the thread will sleep for at most timo / hz seconds. If the timeout
expires, then the sleep function will return EWOULDBLOCK.
Several of the sleep functions including msleep(), msleep_spin(), and the
locking primitive sleep routines specify an additional lock parameter.
The lock will be released before sleeping and reacquired before the sleep
routine returns. If priority includes the PDROP flag, then the lock will
not be reacquired before returning. The lock is used to ensure that a
condition can be checked atomically, and that the current thread can be
suspended without missing a change to the condition, or an associated
wakeup. In addition, all of the sleep routines will fully drop the Giant
mutex (even if recursed) while the thread is suspended and will reacquire
the Giant mutex before the function returns. Note that the Giant mutex
may be specified as the lock to drop. In that case, however, the PDROP
flag is not allowed.
To avoid lost wakeups, either a lock should be used to protect against
races, or a timeout should be specified to place an upper bound on the
delay due to a lost wakeup. As a result, the tsleep() function should
only be invoked with a timeout of 0 when the Giant mutex is held.
The msleep() function requires that mtx reference a default, i.e. non-
spin, mutex. Its use is deprecated in favor of mtx_sleep(9) which
provides identical behavior.
The msleep_spin() function requires that mtx reference a spin mutex. The
msleep_spin() function does not accept a priority parameter and thus does
not support changing the current thread’s priority, the PDROP flag, or
catching signals via the PCATCH flag.
The pause() function is a wrapper around tsleep() that suspends execution
of the current thread for the indicated timeout. The thread can not be
awakened early by signals or calls to wakeup() or wakeup_one().
The wakeup_one() function makes the first thread in the queue that is
sleeping on the parameter chan runnable. This reduces the load when a
large number of threads are sleeping on the same address, but only one of
them can actually do any useful work when made runnable.
Due to the way it works, the wakeup_one() function requires that only
related threads sleep on a specific chan address. It is the programmer’s
responsibility to choose a unique chan value. The older wakeup()
function did not require this, though it was never good practice for
threads to share a chan value. When converting from wakeup() to
wakeup_one(), pay particular attention to ensure that no other threads
wait on the same chan.

RETURNVALUES

If the thread is awakened by a call to wakeup() or wakeup_one(), the
msleep(), msleep_spin(), tsleep(), and locking primitive sleep functions
return 0. Otherwise, a non-zero error code is returned.

ERRORS

msleep(), msleep_spin(), tsleep(), and the locking primitive sleep
functions will fail if:
[EINTR] The PCATCH flag was specified, a signal was caught,
and the system call should be interrupted.
[ERESTART] The PCATCH flag was specified, a signal was caught,
and the system call should be restarted.
[EWOULDBLOCK] A non-zero timeout was specified and the timeout
expired.

SEEALSO

HISTORY

The functions sleep() and wakeup() were present in Version 1 AT&T UNIX.
They were probably also present in the preceding PDP-7 version of UNIX.
They were the basic process synchronization model.
The tsleep() function appeared in 4.4BSD and added the parameters wmesg
and timo. The sleep() function was removed in FreeBSD 2.2. The
wakeup_one() function appeared in FreeBSD 2.2. The msleep() function
appeared in FreeBSD 5.0, and the msleep_spin() function appeared in
FreeBSD 6.2. The pause() function appeared in FreeBSD 7.0.